stack.py

class Stack:

    def __init__(self):
        self.items = []

    def push (self, item):
        """ Accepts an item as a parameter and appends it to the end of the list
        Returns nothing

        The runtime for this method is O(1), or constant time, because appending
        to the end of a list happens in constant time
        """

        self.items.append(item)

    def pop(self):
        """ Removes and returns the last item for the list, which is also the
        top item of the Stack

        The runtime here is constant time, because all it does is index to the
        last item of the list.
        
        """
        if self.items:
            return self.items.pop()

        return None

    def peek(self):
        """ This methods returns the last item in the list, which is also the item
        at the top of Stack

        This method runs in constant time because indexing in the list dones in
        constant time.
        """
        if self.items:
            return self.items[-1]
        return None
    
    def size(self):
        """ This method returns the length of the list that is representing the
        Stack.

        This methods runs in constant time because finding the length of a list
        also in constant time.

        """
        return len(self.items)
        
    def is_empty(self):
        """This method returns a Boolean value describing whether or not the Stack
        is empty.

        Testing the quality happens in constant time.
        """
        return self.items == []